According to FastLaneGaming, there's a P1 GTR coming! (it is NOT the owen's edition because if you look at the back it has that big carbon piece like the real one) :
2019 McLaren 720S Spider | Forza Horizon 5 | kudosprime.com
Horizon Promo in FH4 - FH4 Discussion - Official Forza Community Forums
Forza Horizon 4 | McLaren P1 “Owens” Edition | Full Customization - YouTube
Recent Update for Forza Horizon 4 Vehicles - ForzaTune
NEW* How To INSTANTLY UNLOCK The ALL NEW UNRELEASED McLaren P1 "Owens Edition" in Forza Horizon 4! - YouTube
Fast Lane Gaming - This one is for you, Owen | Disclaimer: Car was only used with an Custom account for an 10 Minute Drive & Shooting. . Car : McLaren P1